The expansion of land plants is considered to have played a key role in triggering the Late Paleozoic Ice Age (LPIA), but evidence linking climatic events to terrestrial floral changes is limited. Here, we generated bulk carbonate δ¹³C, conodont δ¹⁸O and ⁸⁷Sr/⁸⁶Sr profiles from the lowermost Carboniferous of South China and Vietnam in order to investigate their relationship to contemporaneous land plant evolution. Climatic cooling in the mid-Tournaisian coincided with large perturbations to the global carbon cycle and continental weathering regimes as well as with a major diversification episode among seed plants. These relationships are consistent with terrestrial floral changes triggering intensified weathering of basalts (i.e., lower ⁸⁷Sr/⁸⁶Sr), enhanced marine productivity (i.e., higher δ¹³C_carb), and reduced atmospheric pCO₂ and attendant global cooling (i.e., higher conodont δ¹⁸O). The results of our study suggest that expansion of terrestrial floras was a key driver of Early Carboniferous climate change.